Here's the question: are you doing deep litter or just a thick layer of wood chips? If deep litter, I would not add gravel or sand or any other non-organic matter over the soil.
If you need drainage, you'll need to add drains or trench to divert water. My run is unroofed and I get a lot of rain, but I have excellent soil drainage, so the deep litter is ideal for me to get continued good drainage while stabilizing the soil and composting down the poop, reducing smells.
Wood chips do not get mushy from not being aged. The recommendation for aging is due to the fact that fresh chips can in rare cases get deadly mold blooms.
Wood shavings =/= wood chips. Massive difference in how they behave especially for a deep litter set up. Shavings do not allow for the drainage and aeration that chips do.